Clarify the upgrader report

This has the potential to confuse people. If you extend the airbnb
config, you get this warning:

```
* Move ecmaFeatures section under parserOptions section
```

Which I believe originates in:
https://github.com/airbnb/javascript/blob/17917eedfb16ce7124c1d6c8327841d4c4b9bf48/packages/eslint-config-airbnb/rules/react.js#L5-L12
which is extended by the airbnb config. The user has no idea that this
is what it's referring to, and this isn't in their control.

Author: maxjacobson

lib/config_upgrader.js

@@ -247,6 +247,9 @@ class ConfigUpgrader {
       if (report.length > 0 || upgrader.report.length > 0) {
         report = [
           `${bareConfigFilePath} appears to be incompatible with ESLint 3.`,
+          "We've attempted to fix any compatibility issues.",
+          "Note: some of these warnings may originate in configurations you are extending.",
+          "If analysis succeeds, you have nothing to worry about.",
           "To upgrade it do the following:\n"
         ].concat(report).concat(upgrader.report);
       }

test/config_upgrater_test.js

@@ -22,6 +22,9 @@ describe("ConfigUpgrader", function() {
             .upgradeInstructions([directory + '/.eslintrc'], directory);
           expect(report).to.deep.eq([
             ".eslintrc appears to be incompatible with ESLint 3.",
+            "We've attempted to fix any compatibility issues.",
+            "Note: some of these warnings may originate in configurations you are extending.",
+            "If analysis succeeds, you have nothing to worry about.",
             "To upgrade it do the following:\n",
             "* Add .yml or .json to the config file name. Extension-less config file names are deprecated."
           ]);